The present invention relates to an inverter for multi-tube type backlight.
A liquid crystal display panel (LCD) is generally comprised with a backlight as a light source wherein such a backlight is mainly comprised of cold cathode tubes. In case display of high luminance is to be required, a plurality of cold cathode tubes are employed as the backlight for comprising a multi-tube type backlight.
High voltage is required for illuminating cold cathode tubes, and an inverter is used as a light source for illumination. A frequency of a voltage that is supplied to the cold cathode tubes, that is, an oscillating frequency for the inverter generally ranges from 30 to 80 kHz. A step-up transformer for the inverter is mainly used upon one-sided grounding for the purpose of keeping high voltage wirings for connecting outputs of the inverter with the cold cathode tubes short.
A conventional circuit of an inverter for a multi-tube type backlight is illustrated in FIGS. 5, 6 and 7.
In the inverter of FIG. 5, a push-pull type resonance circuit is provided on a primary side of the step-up transformer 11 that is comprised of transistors 7 and 8, a resonance capacitor 9, a choke coil 13 and a primary winding of the step-up transformer 11. Alternating current of high frequency that is generated by this resonance circuit is stepped up by the step-up transformer 11 and is supplied to both cold cathode tubes 3, 4. Since the cold cathode tubes 3, 4 are of negative voltage-current characteristics, ballast capacitors 5, 6 are provided for the purpose of limiting current. One end of a secondary winding of the step-up transformer is grounded so as to achieve so-called one-sided grounding.
The inverter of FIG. 6 is comprised of two step-up transformers 11, 12 that are respectively connected to the cold cathode tubes 3, 4. A primary-side resonance circuit is commonly used by the step-up transformers 11, 12. The step-up transformers 11, 12 are of one-sided grounded type.
Similarly to the inverter of FIG. 6, the inverter of FIG. 7 is also comprised of two step-up transformers 11, 12 that are respectively connected to the cold cathode tubes 3, 4. However, the inverter of FIG. 7 differs from the inverter of FIG. 6 in that separate resonance circuits are provided on primary sides of the step-up transformers 11, 12, respectively. The step-up transformers 11, 12 are of one-sided grounded type.
As explained above, the inverters of multi-tube type backlights utilizing a plurality of cold cathode tubes employ either a method in which a plurality of cold cathode tubes are connected to an output of a step-up transformer (FIG. 5) or a method in which a plurality of step-up transformers are used (FIGS. 6, 7).
In case a plurality of cold cathode tubes are connected to an output of a step-up transformer (FIG. 5), the plurality of cold cathode tubes are supplied with outputs of identical frequency and of identical phase and thus operate in a synchronous manner. In case a common primary-side resonance circuit is used for a plurality of step-up transformers (FIG. 6), the plurality of cold cathode tubes will similarly operate in a synchronous manner. In case the plurality of step-up transformers is respectively provided with primary-side resonance circuits (FIG. 7), the plurality of cold cathode tubes will operate in an asynchronous manner.
However, the following drawbacks are presented in a conventional inverter for a backlight. More particularly, an inverter outputs alternating current of high voltage and high frequency for illuminating cold cathode tubes such that noise resulting from such high voltage will be mixed into control signals or image signals for driving a liquid crystal display panel. It is known that wavelike display noises appear on liquid crystal display panels that are generally referred to as beat noises through interference between high voltage noises generated from the inverter and horizontal synchronous frequencies of the liquid crystal display panel and other factors, wherein sources of generating such noise are high voltage portions, namely the step-up transformers, high voltage wirings, cold cathode tubes, and also lamp reflectors.
As already described, the high voltage outputs that are supplied to the plurality of cold cathode tubes are synchronous in the inverters of FIGS. 5 and 6. Thus, noise N1 resulting from high voltage output 1 of the step-up transformer 11 and noise N2 resulting from high voltage output 2 of the step-up transformer 12 will also be of synchronous waveforms as illustrated in FIG. 8. Because of this fact, composite high voltage noise N will be inputted to the liquid crystal display panel such that beat noises will appear on a display screen.
In the inverter as illustrated in FIG. 7, the high frequency outputs that are supplied to the plurality of cold cathode tubes are not synchronous. Thus, noise N composed of noise N1 from high voltage output 1 and of noise N2 from high voltage output 2 will be similarly inputted to the liquid crystal display panel so that beat noises will appear on the display screen.
A known method for preventing generation of beat noise is one as illustrated in FIG. 10 in which the step-up transformer is made to perform floating operation instead of one-side grounding the same. In the inverter of FIG. 10, output terminals of the step-up transformer 11 are not grounded but connected to both electrodes of the cold cathode tube 3. Similarly, output terminals of the step-up transformer 12 are connected to both electrodes of the cold cathode tube 4. Since high voltage outputs from respective output terminals of the step-up transformers will be of identical frequency but of reverse phase in such an inverter, the composite high voltage noise will be substantially zero. However, in case such an inverter and cold cathode tubes are mounted as actual products, at least one of two high voltage wirings for connecting the step-up transformers and the cold cathode tubes will be a long one. This will lead to an increase in leak current owing to stray capacity of the high voltage wirings to thus undesirably degrade the efficiency of the inverter.
In the cold cathode tube having a smaller diameter and a longer length, the higher the tube voltage becomes, the more beat noise is apt to be generated owing to its characteristics. It is also apt to be generated in case the high voltage wiring is long, in case an interval between the cold cathode tubes and the liquid crystal display panel is narrow, or also in case shielding properties between high voltage portions and the liquid crystal display panel are not sufficient. Such demands are becoming gradually stricter accompanying the tendency of employing a multi-tube type arrangement for backlights in future liquid crystal display panels for achieving further upsizing, thinning and high luminance thereof.
It is therefore an object of the present invention to prevent generation of noise on a display screen owing to secondary-side high voltage of an inverter without increasing lengths of high voltage wirings.
For solving the above problems, the inverter for multi-tube type backlight according to the present invention includes two step-up transformers of one-side grounded type wherein the two step-up transformers respectively output electric power to one or a plurality of cold cathode tubes and wherein outputs of the two step-up transformers are of identical frequency but of mutually reversed phases.
More particularly, in an inverter utilizing a Royer""s circuit, a primary-side resonance circuit is used in common by two step-up transformers of one-side grounded type, wherein outputs of the two step-up transformers are made to be of identical frequency but of mutually reversed phases by setting the two step-up transformers to be of reverse polarity.
Alternatively, two step-up transformers of one-side grounded type are driven in a push-pull manner through identical switching signals and signals obtained by inverting these switching signals, wherein polarities of the two step-up transformers and switching elements into which the switching signals and the signals obtained by inverting these switching signals are inputted are determined such that outputs of the two step-up transformers are of reverse phase.
Moreover, a plurality of inverters each comprised of two step-up transformers that output electric power of identical frequency but of reverse phases are provided for driving and illuminating a plurality of cold cathode tubes.